Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including volatile and non-volatile memory. Volatile memory can require power to maintain its information and includes random-access memory (RAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM), among others. Non-volatile memory can provide persistent information by retaining stored information when not powered and can include NAND flash memory, NOR flash memory, read only memory (ROM), Electrically Erasable Programmable ROM (EEPROM), Erasable Programmable ROM (EPROM), phase change random access memory (PCRAM), resistive random access memory (RRAM), and magnetic random access memory (MRAM), such as spin torque transfer random access memory (STT RAM), among others.
Select devices can be coupled to memory cells. Select devices can be used as a switch, in which the transistor is either fully-on or fully-off. Fully-on has a voltage across the transistor of almost zero and the transistor is “saturated” as it cannot pass any more current. Examples of select devices include thin film transistors (TFTs). TFTs commonly use silicon film. Generally, polycrystalline silicon materials have been widely used as semiconductor materials for TFTs because they have a high field-effect mobility and can be applied to high speed circuits and constitute complementary metal oxide semiconductor (CMOS) circuits. TFTs using polycrystalline silicon materials can be used as active elements of active-matrix liquid crystal display (AMLCD) devices and switching and driving elements of organic light emitting diodes (OLEDs).
Methods of crystallizing an amorphous silicon material into a polycrystalline silicon material include solid phase crystallization (SPC), excimer laser crystallization (ELC), metal induced crystallization (MIC) and metal induced lateral crystallization (MILC). SPC is a method of annealing an amorphous silicon material for several to several tens of hours at a temperature at or below the transition temperature of the glass used as a substrate of a display device employing a thin film transistor (typically, about 700° C. or less). ELC is a method of crystallizing an amorphous silicon material by irradiating the amorphous silicon material with an excimer laser and locally heating the amorphous silicon material to a high temperature for a very short time. MIC is a method of using phase transfer induction from amorphous silicon to polysilicon by contacting the amorphous silicon material with metals such as nickel (Ni), palladium (Pd), gold (Au), and aluminum (Al), or implanting such metals into the amorphous silicon material. MILC is a method of inducing sequential crystallization of an amorphous silicon material by lateral diffusion of silicide formed by reacting metal with silicon.
However, SPC has disadvantages of long processing time and a risk of transformation of the substrate due to the long processing time and high temperature used for the annealing. ELC has disadvantages in the expensive laser equipment is required and interfacial characteristics between a semiconductor material and a gate insulating material may be poor due to protrusions generated on the created polycrystallized surface. MIC and MILC have disadvantages in that a large amount of crystallization-inducing metal remains on the crystallized polycrystalline silicon material to increase the leakage current of a semiconductor material of a TFT.